This invention relates generally to hot melt adhesives and, in particular, to a method of formulating and applying hot melt adhesives in a continuous in-line operation. In one aspect, this invention relates to the formulation and application of an adhesive onto a diaper backsheet.
Hot melt adhesives are blends of several raw materials including polymers, resins, plasticizers and other additives such as antioxidants, fillers, pigments, etc. Since these ingredients must be blended in the proper ratio to obtain the adhesive having the desired properties, it has been the practice of the industry to batch blend the raw materials in a mechanical mixer under controlled conditions to obtain a thoroughly mixed product. The blended product is then discharged as an amorphous melt, collected, and packaged for storage and shipment to the end user. The end user transfers the solid formulated adhesive to a melt tank for feeding a hot melt applicator. This approach is inefficient and costly for the following reasons:
(a) the adhesive must be remelted at the location of use, requiring energy and specialized melting equipment; PA1 (b) the adhesive must contain an antioxidant to prevent degradation of the adhesive during the remelting and application process and during long periods of storage; PA1 (c) the formulation cannot be modified at the point of use; PA1 (d) the quality of adhesive may suffer during storage or transport due to polymer degradation or contamination; PA1 (e) plasticizers are required to ensure viscosity stability during the remelt and application process. PA1 (a) continuously metering into the barrel of an extruder raw materials Comprising particles of a polymer adhesive and a resin in the proper weight ratio; PA1 (b) heating and mixing the raw materials in the extruder to form a homogeneous melt of the raw materials; PA1 (c) continuously discharging the melt from the extruder directly into an applicator system comprising a plurality of applicators, each applicator having a pump and an applicator head; PA1 (d) discharging the melt from the applicator heads onto a substrate.
In certain operations where large volumes of adhesives are used, such as in label coating, tape manufacture, or diaper chasis, the operator sometimes employs an in-line system comprising a batch mechanical mixer (e.g. Banbury mixer). In this process, the adhesive ingredients are batch mixed in the mixer until a homogeneous adhesive formulation (typically 2,000 to 5,000 pound batches) is obtained. The mixing is then discontinued and the adhesive formulation is discharged from the mixer to a pump and then pumped through heated lines to a roll coater for application on a coated stock. The present invention offers significant advantages over the batch prior art process, as well as the fully formulated approach described above. The continuous operation of the present invention ensures uninterrupted operation of the line and eliminates the packaging and shipping costs. Moreover, the continuous process is especially adapted to air assisted adhesive applicators, which is the preferred embodiment of the present invention.